This invention relates to an improved method and apparatus for translating data representations on a rotating disc.
With conventional translating techniques data representations are translated on a rotating disc by a transducer which moves to different positions along a radial arc of the disc. As a result data is translated on concentric circular tracks. The tracks on the disc surface area included within any given angular rotation of a radius are known as sectors and the track length within any given sector is a function of the radius of the track. The rate at which data is supplied to the transducer is called the data rate. The data rate is typically constant for a given system. Because of the critical role of timing in high speed, high density translating of data, the rotational speed of the disc, called spindle speed, is also constant.
Fixed data rate and spindle speed together with disc geometry have traditionally dictated an inefficient distribution of data on the disc. Track data density, i.e., bits of data per unit of track length, is lowest in the outermost tracks. As track diameter decreases track data density increases until it reaches an operational limit within the translating system. At this point, the remaining inner portion of the disc is left unused. Once the innermost track radius and the data density of the innermost track are established the density of data in each track having a greater radius varies inversely with the ratio of the radii of that outer track and the innermost track. Or, stated another way, with both a fixed data rate and a fixed spindle speed each track receives the same quantity of data with track data density dependant upon the circumference of the track. Thus, outer diameter tracks are not utilized to their potential capacity for storing data.
A central concern to disc translating systems is maximizing the data stored per disc with the ultimate goal being reduction in the cost of storing data. With conventional disc translating techniques net data per disc surface is maximized when the diameter and hence track length, of the outermost track is twice that of the innermost track. At this ratio the data density of the outermost track is one half that of the innermost track. Assuming that the innermost track is always translated at a data density equal to the acceptable operational limit of the translating system, data storage gained by the addition of tracks having smaller diameters is out weighed by the data storage lost to lower data density in the longer outer diameter tracks. Thus, the quantity of data stored on a disc surface could be increased if track data density could be controlled to more closely approach the maximum data density for each track of the disc surface.